Function serialize_tx_for_fund_raw

pub(crate) fn serialize_tx_for_fund_raw(tx: &Transaction) -> Vec<u8> 

Serialize a transaction for fund_raw_transaction, working around Bitcoin Core’s deserialization bug for 0-input segwit transactions. fund_raw_transaction RPC gives deserialization error for 0-input transactions with segwit flag.

For transactions with no inputs, this uses legacy-style serialization (version, inputs, outputs, locktime) without segwit markers. Core will then add inputs and return a proper segwit transaction.